Due to the general necessity to reduce CO2 emissions and to the loss of trust in nuclear power plants, devices for producing electricity with low temperature sources are gaining of importance. Low temperature sources include e.g. industrial waste heat, low temperature geothermal heat sources, low temperature biomass energy and low temperature solar energy, but also novel low temperature heat generators based on chemical or nuclear reactions.
Such devices generally work according to a so-called Organic Rankine Cycle (ORC), i.e. a Rankine cycle in which the working fluid is an organic fluid with a lower evaporation temperature than water. The working principle underlying the ORC is basically the same as that of the classical Rankine cycle in which the working fluid is water. However, due to the lower evaporation temperature of the working fluid, the external heat source in an ORC may be in a lower temperature range than the external heat source in a Rankine cycle working with water.
Today, devices for power generation according to an ORC are commercially available mainly in a range starting with 0.3 MW electric power output, but there is an increasing need for such devices with a smaller electric power output too. In particular for the range of 25 kW to 250 kW electric (corresponding to a thermal recuperation range of about 150 kW to 1.5 MW), there seem to be interesting applications for producing electricity with low temperature sources using an ORC. However, with a decreasing nominal power of the device used for power generation, the investment costs per kW installed strongly increase and the efficiency of power generation decreases.
If the external heat source is connected into the ORC by means of a heat carrier medium, which has to be cooled down in an evaporator working as counter-current heat exchanger, it is known to operate an ORC with more than one evaporator. Each evaporator then works at a different evaporation pressure, i.e. with a different evaporation temperature, in combination either with a separate expansion machine for each evaporator or with a single multi-stage expansion machine, in which the vapour produced in each additional evaporator, is injected into an intermediate stage of the multi-stage expansion machine. Due to the fact that the heat transfer is split between evaporators working at different evaporation temperatures, one can work with a more important temperature differential on the side of the heat carrier medium, i.e. transform more heat into power.
ORC systems with more than one evaporator are e.g. described in DE 10 2007 044 625 A1 . According to a first embodiment, the system comprises several separate ORCs, each of these ORCs comprising an evaporator, a turbine, a condenser and a condensate pump. With regard to the heat carrier fluid, the evaporators are basically connected in series. With each evaporator is associated a turbine comprising its own housing with a nozzle system and blade wheels. These turbines are regrouped in pairs, wherein the blade wheels of a turbine pair have a common shaft. The parallel shafts of two turbine pairs are interconnected by a gear system to drive an electrical generator. Such a multi-turbine solution is of course expensive and cumbersome.
According to a second embodiment described in DE 10 2007 044 625 A1, the system comprises two evaporators associated with a two-stage turbine. This two-stage turbine comprises a rotor carrying two axially spaced blade rings, wherein the first blade ring has a smaller diameter than the second blade ring. A first steam flow (i.e. high pressure steam produced by a high pressure evaporator) radially enters into the turbine housing through a high pressure inlet and flows through a first annular channel radially into a first annular nozzle ring, which deflects the flow in an axial direction into the first rotor blade ring, i.e. the blade ring with the smaller diameter. A second steam flow (low pressure steam produced by a low pressure evaporator) radially enters into the turbine housing through a low pressure inlet and flows through a second annular steam channel into a second nozzle ring, which deflects the flow in an axial direction into the second rotor blade ring, i.e. the blade ring with the bigger diameter. The two stages are designed so as to achieve the same end pressure at the outlet of the first and second blade ring, wherein the exhaust streams are only merged in an outlet diffuser of the turbine. It is obvious that such a turbine has a rather low efficiency, when compared e.g. to a typical induction type turbine, i.e. a multi-stage axial turbine in which low pressure steam is induced into the main vapour stream at an intermediate turbine stage and both streams are thereafter commonly expanded. However, for power generation with an ORC in the kW-range, known induction type turbines are by far too expensive.
GB 403,335 and U.S. Pat. No. 1,870,212 show a radial-outward-flow type multi-stage turbine, with an axial main vapour inlet port and an annular secondary vapour inlet port, which is arranged in the turbine so as to annularly induce, in an intermediary stage of the turbine, a secondary vapour stream into an already partially expanded radial main vapour stream. The main vapour inlet port and the annular secondary vapour inlet port have to be separated by complicated labyrinth packaging, which makes the turbine rather expensive.
DE 537,917 shows a rather complicated design of a radial-flow type turbine, in which the rotor comprises axially spaced sets of stator/rotor assemblies, which are separated by separation walls and connected either in parallel or in series.
An object of the present invention is consequently to provide an induction type turbine, which can be produced at relatively low costs, and which has nevertheless good efficiency, so as to be e.g. an interesting solution for power generation with an ORC below 1 MW electric.